Traditional methods of ensuring correctness when rendering large three-dimensional volumetric models require the sorting of particles in order to draw an image (e.g., painter's algorithm). This type of sorting is view dependent and generally has been either too expensive to allow for real-time rendering, or has required continuous re-sorting as the view changed, with various incremental approaches to sorting particles, including graphics processing unit (GPU) sorting, clustering, and so on. This continuous resorting of large scale 3-D volumetric models is also computationally expensive.
Thus, large 3-D volumetric model navigation, such as navigation in and around medical or interstellar 3-D volumetric models, is not possible in real time on a personal computer or comparable device that includes typical off-the-shelf graphics processors. Current applications that allow navigation of large 3-D volumetric models having emissive and occlusive particles provide only previously recorded guided tours. For example, users must choose from a growing number of guided tours created by astronomers and educators from famous observatories and planetariums.